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• When is the callback function of avio_alloc_context() triggered as the official ffmpeg documents ?

  2 septembre 2021, par user14767109

  According to the official FFmpeg documents (such as the two links below), the fifth parameter of avio_alloc_context() function is the callback function. How can this callback function be triggered ?

  


  I don't understand the official documents very well.

  


  AVIOContext* avio_alloc_context(unsigned char * buffer,
    int     buffer_size,
    int     write_flag,
    void *  opaque,
    int(*)(void *opaque, uint8_t *buf, int buf_size)     read_packet,
    int(*)(void *opaque, uint8_t *buf, int buf_size)     write_packet,
    int64_t(*)(void *opaque, int64_t offset, int whence) seek
)


  


  Function link : function link

  


  Code link：code link

  


• ffmpeg drawtext to 'burn' text to video

  13 janvier 2018, par itsme

  I made a video in .MOV format which I edited until I got a mp4 file. Instead of subtitles, I used multiple drawtext feature (in a bash batch file) of ffmpeg to add texts.

  In a China Weibo post (like twitter), I link to my uploaded video in Youku. If I click on the youku link, it brings me to youku and the video plays with the text. When I click on the video image of Weibo (I supposed hosted video or live streaming , etc ?) the video plays but without the text from drawtext.

  Is there a way I could redo my drawtext (with some settings) so that the text could not be removed by weibo ?

  [edit]
  Very sorry. My mistake. I am new to Weibo with only 1 post. Weibo gives an image for preview of any video link. I think it is limited to 6 min 30secs. My drawtext starts only after.

• FFmpeg.wasm demuxing - Get encodedChunks in Javascript

  16 mars 2023, par Kevin Baving

  I am building a video editor whose process looks like this :

  


  Demuxing -> Decoding -> Editing -> Encoding -> Muxing.

  


  The demuxing and muxing process is currently done with mp4box.js. I would like to replace mp4box.js with ffmpeg.wasm. Unfortunately, I can't get along with the process.

  


  What should FFmpeg.wasm do in the demuxing process ?

  


  

  • load a .mp4 file

  


  • extract the encodedVideoChunks and store them as EncodedVideoChunk objects in an array

  


  • extract the encodedAudioChunks and store them as EncodedAudioChunk objects in an array

  


  • get some metadata like : duration, timescale, fps, track_width, track_height, codec, audio_channel_count, sample_rate ....

  


  


  public async loadFile(file: File) {
    let data = await fetchFile(file)
    let blob = new Blob();
    await this.ffmpeg.setProgress(({ratio }) => console.log(`Extracting frames: ${Math.round(ratio * 100)}%`));
    this.ffmpeg.FS('writeFile', 'videoTest.mp4', data);
    //Here is where I am struggling
    //Should look like this: 
    //const command = '-i videoTest.mp4 -c:v copy .... '
    //await this.ffmpeg.run(command);
    //....
}


  


  Lets get deeper into my problem :

  


  Because FFmpeg.wasm is still a cli tool, I have no idea what the best way to safe the encodedChunks into a file is (and what kind of filetype I should use). Further I would like to know how to read that file propertly so that i can safe the input of the file into seperate EncodedVideo- and AudioChunks.